1. Field of the Invention
The present invention relates to check valves and, more particularly, to check valves for use with leak detectors in retail gasoline dispensing systems.
2. Description of the Prior Art
Gasoline dispensers used at automotive service stations dispense gasoline from an underground tank through a nozzle placed in the fill tube of an automobile gas tank. The underground tank includes a pump actuated by a user upon manipulation of a lever at the time of lifting the nozzle from its stored position on the dispenser. Downstream of the pump is a conventional leak detector, which may be of the piston or diaphragm type. For example, a diaphragm leak detector long used since 1976 is known as a Red Jacket Two-Second Model No. 116-017 Leak Detector; a similar unit has been used since 1959. These units are manufactured by the Marley Pump Company. The gasoline line downstream of the leak detector may have a leak which creates an environmental hazard to the adjacent underground and a severe safety hazard to personnel. Such leaks are difficult to detect from the surface of the ground and the leak detector serves the function of precluding delivery of gasoline through the delivery line if a leak of greater than a predetermined rate is present. Trapped air in the delivery line usually does not present a safety hazard but it may have the effect of extending the leak sensing function of the leak detector to the irritation of a user attempting to dispense gasoline. Changes in environmental temperatures may expand or contract the fluid (product) within the delivery line which will vary the pressure in the delivery line and have an effect upon the operation of the leak detector. Upon closing of the dispensing nozzle, a pressure wave rebounds from the dispensing nozzle upstream through the delivery line to the leak detector. This pressure wave, often referred to as hydraulic hammer, subjects the leak detector to potential for severe damage of critical parts, including rupture of the diaphragm in a diaphragm type leak detector. Such damage may be undetected for an extended period of time. If leaks in the delivery line ar present or result from the damaging effects of hydraulic hammer, the damaged leak detector would be incapable of serving the function of detecting such leaks. The resulting environmental and safety hazards would continue unchecked. Hydraulic hammer also causes the leak detector to open prematurely because of the resulting excess pressure down stream of the leak detector. Just turning on the pump, without actuating the nozzle, may cause hydraulic hammer to a detrimental extent.